NFSv4.2: xattr cache: remove unused cache struct field

The hash_lock field of the cache structure was a leftover
of a previous iteration of the code. It is now unused,
so remove it.

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>